NetStumbler is a free packet sniffer
utility used to locate Wi-Fi networks.
It's one of the first tools of its
kind and arguably still the best. So
much so that it is the standard
product used by war-drivers in their
perpetual search for free bandwidth
via unsecured Wi-Fi networks as well
as road-warriors looking to connect
their laptops back to the office.

I've used the freeWireless Net View from nirsoft for ages now, works on XP, Vista, Windows 7 absolutely fine (works on my Win7 64 bit), and needs no installation you just run the exe file, so it's a great one to keep on your USB key of utilities